In nature electric signaling is based on ionic conductors. The human brain sends electrical impulses (action potentials) through the axons of neurons to control muscle contraction. Sensory neurons convert external stimuli like pressure or temperature changes into electric signals that are sent to the brain—all mediated by the movement of ions. In contrast to nature modern electrical devices are largely based on electronic conductors, mainly metals. Distributing electrical power for lighting, heating and powering machines necessitates high current densities where metals like copper, aluminum, silver or gold excel with superior conductivity. The free electrons in the conduction band of metals allow for ultra-high frequencies in modern computation and communication devices.
Inspired by an increasing interest to extend the use of electronics to circuits on flexible and stretchable substrates (sensor skins for soft machines), interfacing electronics with life, research has yielded ingenious solutions to make metal stretchable. Most commonly used are combinations of elastomeric substrates with gold/copper/etc. conductors on top that form meanders, buckling wave structures, and bridges. Also used are conducting particles inside an elastomeric matrix where conductivity is based on percolation.
Nonetheless, metals are problematic when conductors are required to be transparent. Indium tin oxide (ITO) is used for many applications, where flexibility is not required. Carbon nanotubes, silver nanowires and graphene on elastic (deformable) substrates have been developed for applications that require mechanical compliance (flexible and stretchable displays, tunable optics). Stretchable conductors usually show a steep increase in resistivity with stretch due to percolation effects. Current candidates for transparent electrodes have to balance transparency and sheet resistance.
Stretchable conductors used as electrodes for dielectric elastomer actuators have very demanding requirements. For example, the dielectric elastomer should remain conductive at large strains. Efforts to make stretchable, transparent solid-state electrodes have been confined to electronic conductors, with limited success. Compliant electrodes are commonly made of carbon grease; consequently, the transducers are greasy and opaque. Other conductive materials having higher transparency, for example, indium tin oxide (ITO. carbon nanotubes, silver nanowires and grapheme layers in or on the surface of elastomeric structures have been shown to combine transparency with stretchability. These solutions have one thing in common: the electrical resistivity strongly increases with stretch due to disruption of the conductive pathways in the stretchable conductor matrix—percolation in the case of nanotubes and nanowires and fracture in the case of graphene. Clearly, there is conflicting demand: increasing the amount of conducting particles decreases resistivity but at the same time transparency is also affected negatively due to the augmented amount of light scattering or absorbing particles. No solutions are available today for low sheet resistance at large strains paired with high transparency (for many industrial applications it is necessary to have transmittance >90%.
Our skin—a principal site to perceive the world—is a platform for stretchable, large-area sensors. This design in nature has inspired the development of “electronic skin”. Existing designs of electronic skin rely on electronic conductors, which struggle to achieve high stretchability, along with attributes required in specific applications, such as biocompatibility in medical devices, and transparency in tunable optics. It has been challenging for these electronic conductors to meet various demands associated with sensor skins, such as high stretchability, biocompatibility, and transparency. By contrast, nature has chosen ionic conductors—nerves—to sense and distribute signals throughout our bodies.